An active pixel sensor (“APS”) is a special kind of light sensing device. Each active pixel includes a light sensing element and one or more active transistors within the pixel itself. The active transistors amplify and buffer the signals generated by the light sensing elements in the pixels. One type of such APS devices is disclosed in U.S. Pat. No. 5,471,515 by Fossum et al., the disclosure of which is incorporated herein by reference.
There are many applications for active pixel image sensors, including scientific, as well as commercial and consumer applications. The special techniques of active pixel sensing allow using a semiconductor family formation process which is compatible with CMOS, e.g., NMOS. This technique enables the readout electronic to be integrated on the wafer using a similar process. The result is a high performance sensor with high quantum efficiency and low dark current.
CMOS active pixel image sensors may be operated using a “rolling” shutter. Such a shutter operates by reading out each row of pixels, and then resetting that individual row, and then rolling to read and then reset the next row of pixels. Each pixel hence gets read and then reset at slightly different times. Hence, each pixel has a slightly different time of integration. Some applications, such as high-speed photography, may require more time consistency than is possible using this approach. Therefore, in these other applications, a frame shutter may be used. In the frame shutter mode, all pixels in the array have substantially identical integration start times and integration stop times.
A wide dynamic range (WiDyR) technique was developed for CMOS sensors with a rolling shutter and is described in U.S. Pat. No. 6,115,065, which is incorporated herein by reference. The WiDyR technique allows the extension of the intra-scene dynamic range of images by combining an image taken with a short exposure time with an image taken with a long exposure time. U.S. Pat. No. 6,115,065 teaches designs and operational methods to increase the dynamic range of image sensors and APS devices in particular by achieving more than one integration times for each pixel thereof. An APS system with more than one column-parallel signal chains for readout are described for maintaining a high frame rate in readout. Each active pixel is sampled for multiple times during a single frame readout, thus resulting in multiple integration times. The operation methods can also be used to obtain multiple-integration times for each pixel with an APS design having a single column-parallel signal chain for readout. Furthermore, analog-to-digital conversion of high speed and high resolution can be implemented.